1. Field of the Invention
The present invention relates to system identification in radio communications systems such as cordless telephone and radio paging systems.
2. Description of the Related Art
Communications systems will increasingly have to share an environment due to external constraints such as the band of frequencies which are allocated by governmental authorities for certain applications. Where several users are having to share an environment then it is necessary that each user can be distinguished from the others. Hence some form of identification is necessary.
In radio paging, identification signals are frequently transmitted by a base station at regular intervals on a channel allocated for signalling. For paging purposes this channel may be the only channel used by the system or, in more extensive systems, it may be used just for establishing initial contact. In the past these systems have usually been designed with only one base station and in consequence there is no likelihood of clashes between different systems. However, in the case of several communications systems sharing a band of frequencies, clashes may occur if a base station belonging to another user also sends identification signals at the same instants as the first-mentioned base station on the same signalling channel.
A method of avoiding this problem is for each base station sharing a channel to transmit their signalling information at different time intervals together with a base station identification signal. Each base station and each paging receiver associated with that base station has to store these time intervals and the base station identification in order (i) to know when signals are being sent and (ii) to be able to identify the base station.
In radio communications systems such as paging and cordless telephones, the paging receiver and telephone handset are powered by batteries. To extend the life of the batteries when the receiver/handset is on standby the main receiver section may be switched off apart from the intervals when it is signalled. Typically, the signalling of identification and other information may be transmitted for one period in every hundred periods, say 5 mS in 500 mS. Additionally, when a base station is actually calling a receiver/handset then it should be able to convey all the required information as rapidly as possible. However, because of the receiver 1:100 duty cycle it may be necessary to wait for up to one complete interval between signals before any information can be sent. It is therefore desirable to keep this signalling time short. The period for the transmission of the identification and other information is determined by the amount of information which has to be transmitted and hence by reducing the amount of identification information to be transmitted then the overall frequency of the signalling can be increased.